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Why is my Cisco switch stuck in a boot loop after the January 2026 DNS bug?
Critical Advisory: Cisco Switch Boot Loops and DNS Failure
Network administrators managing Cisco Small Business infrastructure are currently facing a widespread critical failure. Beginning January 8, 2026, specific switch models began entering continuous boot loops. This behavior stems from a logic error within the firmware’s DNS client during hostname resolution attempts.
Technical Analysis of the Error
The crash occurs when the affected device attempts to resolve the domain www.cisco.com or specific NTP time servers. Upon this resolution attempt, the system fails to handle the response correctly, logging a fatal exception before initiating a self-protective restart. This creates an infinite loop of booting, attempting resolution, crashing, and rebooting.
Logs from affected devices consistently display the following critical alert immediately prior to the crash:
%DNS_CLIENT-F-SRCADDRFAIL: Result is 2. The address for the specified name "www.cisco.com" could not be identified...
This error manifests regardless of the site’s geographical location or internet connectivity status. Even isolated networks report the issue, indicating the trigger is internal to the switch’s scheduled tasks rather than an external attack.
Affected Hardware
Current reports and community documentation identify the following series as vulnerable:
- Cisco SG500 & SG550X Series
- Cisco CBS Series (specifically CBS250)
- Cisco C1200 Series
Immediate Remediation Strategy
To restore network stability, you must disable the device’s ability to perform DNS lookups. This prevents the faulty code path from executing.
Step-by-Step Workaround:
- Access the switch via the console cable (as SSH/Telnet may be unstable during the loop).
- Enter global configuration mode.
- Execute the following command:
text no ip domain lookup
- Save the configuration to the running startup config.
This command stops the switch from attempting to resolve hostnames, effectively bypassing the bug and stabilizing the uptime. While this disables feature dependencies relying on DNS, it restores packet switching and management access immediately.